The present invention relates to circuit protection and more particularly to fuse protection. Miniature cartridge fuses commonly include a main insulating housing, conductive end caps secured to the housing and a fuse element or wire extending across the end caps. The cup-shaped or open end caps include a skirt portion that extends over the ends of the housing. The fuse element may be electrically and physically secured to the end caps via a body of solder in each of the end caps. The solder can also extend into small clearance spaces between the skirt of the end caps and the electrically insulative housing.
Miniature or subminiature cartridge fuses are typically soldered directly to a printed circuit board (“PCB”) via a process called reflow soldering. The miniature fuse is typically picked and placed onto conductive pads of the PCB, which have been coated or printed with a solder paste. The PCB carrying the fuse is sent through an oven called a reflow oven. The reflow oven heats the PCB and fuse to a temperature that melts the solder paste beneath the miniature fuse. The solder paste melts or reflows at a temperature of, e.g., 180 to 186° C. as the PCB travels through the oven. Afterward, the PCB and associated components cool, allowing the solder paste to harden and fix the miniature fuse to the PCB.
Larger surface mounted or PCB fuses can be placed in clips that are soldered to the PCB. Prior art FIG. 1 shows one example of a pair of such clips 12, which are manufactured by the assignee of the present invention. The fuse can be, e.g., a ¼ inch (6.35 mm) fuse that is clipped into spring-like walls 14 and 16 of clips 12. Clips 12 are spaced apart on the PCB so that walls 14 and 16 extend around and connect electrically with the end caps of a replaceable fuse.
Clips 12 are soldered to the PCB via an alternative process called wave soldering. Here pins 18 extending downwardly from walls 14 and 16 are fitted into holes formed in the PCB. The PCB is sent through a machine called a wave soldering machine, which can have one or more waves of flowing solder. The solder from the waves wicks up through the holes in the PCB into which pins 18 are inserted as the PCB is conveyed over the one or more waves. After the PCB passes the solder bath, the solder cools and creates solder joints holding clips 12 to the PCB. The fuse may then be inserted into clips 12.
Prior art FIGS. 2 and 3 illustrate another surface mountable fuse holder, which is also manufactured by the assignee of the present invention. Here, a plastic housing 40 is provided in assembly with a fuse 42. Plastic housing 40 includes side walls 44 and end walls 46. Terminals 48 are attached to housing 40. Terminals 48 communicate electrically with the end caps of fuse 42 and are soldered to the PCB via the above-described reflow soldering process. The fuse end caps and clips have surface finishes which facilitate solder attachment of the clips to the circuit board pads, but will not reflow the fuse end caps to the clips during the soldering operation. Fuse 42 is replaceable as is the fuse used with clips 22 of FIG. 1.
Both reflow and wave soldering form solder joints between conductive pads provided on the PCB and the directly mounted fuses or the fuse clips. The solder and pads act as a heat sink, potentially preventing the fuse from reacting as quickly as desired to an overcurrent condition because heat generated by self-heating of the fuse element in response to the overcurrent condition is conducted away from the fuse element via the end caps to the circuit board pads and traces, either via the clips, or directly in the case where the fuse is attached directly to the PCB. This conductance of heat away from the fuse element can prevent the element from reaching its melting temperature, thus interrupting the overload condition as intended.
Further, as discussed above, some surface-mounted fuses are mounted directly to the PCB. If such fuses open, the PCB must be reworked, which entails diagnosis, removal of the PCB from the application, possible shipping or transport of the PCB and potentially harmful hand-soldering to remove and replace the fuse.
Accordingly, a need exists for an improved surface mount fuse assembly.